Polytetrafluoroethylene (PTFE) fine powder is the type of PTFE that is made by aqueous dispersion polymerization, followed by coagulation of the dispersion, and drying of the resultant coagulum to obtain the fine powder. Because the PTFE fine powder does not flow in the melt condition (m.p. 327.degree. C.), it has been fabricated by an extrusion method which does not require melt flow.
This extrusion method is known as paste extrusion and is described for example in U.S. Pat. No. 2,685,707 (Llewellyn and Lontz). In paste extrusion, the paste extrusion composition is formed by mixing the PTFE fine powder with an organic lubricant which has a viscosity of at least 0.45 centipoises at 25.degree. C. and is liquid under the conditions of subsequent extrusion. The PTFE soaks up the lubricant, resulting in a dry, pressure coalescing mixture, which is the paste extrusion composition, or simply, the lubricated PTFE fine powder. The lubricated PTFE fine powder is placed in an extrusion barrel which terminates in an extrusion head which defines an extrusion orifice. The powder is then "paste extruded" by a ram positioned at the opposite end of the extrusion barrel moving towards the extrusion orifice to force the lubricated PTFE fine powder through the orifice in such form as sheet, rod, tubing, or coating such as on a wire fed axially along the extrusion barrel and through the center of the orifice. The paste extrusion has usually been carried out at a temperature of 20 to 40.degree. C. In most cases, the paste extrudate is then heated to volatilize the lubricant, usually to a temperature of 100 to 250.degree. C., to drive off the lubricant from the extrudate, followed by sintering of the PTFE.
Example 1 of the '707 patent discloses the paste extrusion of the lubricated PTFE fine powder fed as a powder to the extrusion barrel, followed by calendering the paste extrudate to form tape. Example 2 discloses the paste extrusion of lubricated PTFE fine powder fed as a preform to the extrusion barrel, i.e. the lubricated PTFE fine powder is pre-compacted in a mold into a shape which is complementary to the extrusion barrel. After loading the preform into the extrusion barrel, the ram forces the preform towards and through the extrusion orifice into the extruded shape desired. This has been the method used to form paste extrudate which is not subsequently calendered. Thus, U.S. Pat. No. 4,576,869 (Malhotra), which discloses a highly advantageous PTFE fine powder for paste extrusion, discloses the preforming of the lubricated PTFE fine powder before paste extruding it.
Paste extrusion using the preform method is necessarily a batch or discontinuous extrusion operation. When the preform is used up, the extrusion must be stopped, the ram retracted, and another preform loaded into the extrusion barrel. The trailing end of the preceding preform is still in the extrusion barrel, just upstream from the extrusion head, and the leading end of the succeeding preform is forced into contact with the trailing end, whereby there is a junction between the two preforms within the barrel. Exertion of the ram against succeeding preform causes this junction to be paste extruded. Unfortunately, this junction does not knit together very well in the paste extrudate and this is not cured by sintering. Calendering cannot be used when the shape of the extrudate is the shape desired for the final product, e.g. rod, tubing, and coated wire. Thus for any extrudate applications, the continuity of the extrudate has to be interrupted where the junction occurs in the paste extrudate. The same is true when the powder feed method to the extrusion barrel is used and the extrudate is not calendered. The pressure of the ram compacting the powder also forms a junction with a subsequent batch of lubricated PTFE fine powder, and this junction does not have the integrity of the surrounding paste extrudate.
The need exists for paste extrudability of PTFE lubricated fine powder wherein the junction between batches of the fine powder fed to the extrusion barrel has integrity in the resultant paste extrudate, without requiring calendering. If this need could be satisfied, the paste extrudate could be continuous. U.S. Pat. No. 3,260,774 discloses continuous extrusion of PTFE fine powder using an extrusion screw and special temperature profile in the extruder, but this process suffered from the difficulty in maintaining the special temperatures required, and the action of the screw on the fine powder injured the fragile fine powder, giving inferior extrudate, with the result being that this process is not believed to be in use.